Optical switches are very important devices in optical networks. They are used for network protection, cross connection, add/drop applications, etc. There are many kinds of optical switching devices, including mechanical, electro-optic, thermo-optic, acousto-optic, magneto-optic, and semiconductor. Each switching technology has its own advantages, but typically has drawbacks as well. Mechanical switches are currently the most widely used routing components and provide very low insertion loss and crosstalk characteristics. But their switching times are limited to the millisecond range and they have large sizes. Moreover, due to the use of motor-driven parts, they have limited switch lifetime and thus present reliability issues.
Various attempts have been made to overcome the problems associated with mechanical switching. Most notably, various U.S. patents disclose optical switches that use birefringent walk-off crystals and polarization rotators to perform optical switching. For example, U.S. Pat. No. 6,173,092 discloses an optical mirror switch using a pair of walk-off crystals, a Faraday rotator, and a mirror. U.S. Pat. No. 6,360,034 discloses a reflection-based optical switch that uses Faraday rotators and walk-off crystals. U.S. Pat. No. 5,724,165 discloses in FIGS. 4a and 4b a reflective optical switch that uses two walk-off crystals and a polarization rotator array. Common to the design of these three patents is the use of two walk-off crystals. As a result of this design, the switched optical beams exiting the devices are separated by a walk-off displacements. In other words, the beams in the two switched states approach the exit fibers along optical paths that are parallel to each other. Because the beams are parallel, multiple collimating lenses are required to collect the beams and direct them into their respective fibers. Consequently, these switches need several single-fiber pigtails to couple fibers into and out of the device.